DNA replication, one of the most fundamental processes of all living cells, remains one of the great unsolved problems of molecular biology. The proposed research is concerned with the mechanism of DNA replication in both bacteria and mammalian cells. The approach will be three-fold. First, we will continue our studies on the mechanism of action of the DNA synthesis inhibitor, nalidixic acid. Second, we will study the mechanism of DNA chain growth in both Bacillus subtilis and bacteriophage T7-infected cells. Attention will focus on the initiation, synthesis and structure of nascent short DNA chains, the so- called Okazaki pieces. Third, we propose to study DNA replication in nuclei and in concentrated nuclear lysates from mouse liver cells and from normal and tumor virus-transformed mouse fibroblast cells. Since normal cells are thought to undergo DNA synthesis during transformation to tumor cells, a comparison of in vitro DNA replication in normal and transformed cells may shed further light on the cancer problem.